You are a customer retention and email marketing expert. Write a detailed, 2000-word blog post with 10 product recommendation-based re-engagement email templates. For each template, include: 1) Inactivity period, 2) Recommendation logic (past purchases, browsing), 3) Subject line options, 4) Email body with personalized recommendations, 5) Call-to-action buttons, 6) Personalization, 7) Metrics to track (click rate, re-engagement), 8) A/B testing ideas, 9) Examples, 10) Data usage.